The invention relates to a method for setting a service level in a digital mobile communication system, and to a digital mobile communication system.
The quality of the service offered to a mobile communication subscriber within a mobile communication network is determined by the joint effect of several factors. In the present application quality of service is understood to mean the technical quality of a radio connection, e.g. the transmission rate and the accuracy of the information transmitted, and the accessibility of the radio channel at a desired moment. The term service level is used to refer to the level of the above quality of service.
Within a digital data communication system, transmission errors deteriorating the quality of the signal transmitted occur on a transmission path during speech or data transmission. On the radio path transmission errors occur when a signal encounters interference e.g. due to multipath propagation, an interference signal or a high level of background noise. Channel coding of the digital signal to be transmitted improves the quality of transmission and the tolerance for transmission errors. In channel coding, redundancy is added to a coded speech or data bit sequence by means of error check bits calculated from the original signal. In a receiver, channel coding is decoded in a channel decoder, which allows using error check bits for detecting or even correcting errors occurring during transmission.
Channel coding increases the number of bits transmitted. For instance, when error check bits with a transmission rate of 9.8 kbit/s are added to a full rate speech signal of 13 kbit/s in a GSM system, the total transmission rate is 22.8 kbit/s. The level of protection provided by channel coding is arranged according to need. For rapid transmission of a large quantity of data, channel coding is reduced in order to allow more payload data to be transmitted on the transmission channel. Channel coding may be arranged to detect well and also to correct errors occurring during transmission, or to detect errors only. Channel coding may also be left out altogether. In this case the transmission is totally unprotected against transmission errors. In the GSM system, the bits to be transmitted are classified according to their significance into different groups, where channel coding is effected at a predetermined level. The problem with this solution is that efficient channel coding of the most important bits always reserves transmission capacity on the radio path. Transmission errors do not always occur during transmission. In such cases channel coding has unnecessarily blocked radio channels. On the other hand, unprotected high-speed information transmission causes problems in an environment subject to interference.
FIG. 1 of the attached drawing presents a simplified block diagram of the Pan-European GSM mobile communication system. A Mobile Station MS is connected over the radio path to a Base Transceiver Station BTS, which in the case of FIG. 1 is base station BTS1. A Base Station System BSS comprises a Base Station Controller BSC and base stations BTS controlled by it. Usually a Mobile Services Switching Centre MSC controls several base station controllers BSC. A mobile services switching centre MSC has an interface to other mobile services switching centres and, via a Gateway Mobile Services Switching Centre GMSC, to a public telephone network. The operation of the whole system is controlled by an Operation and Maintenance Centre OMC. Subscriber information concerning a mobile station MS is permanently stored in the system""s Home Location Register HLR and temporarily in a Visitor Location Register VLR within the area of which the mobile station MS is currently located.
Cellular mobile communication systems use several slightly over-lapping radio cells for producing radio coverage. In radio network planning, cell coverage in areas requiring high traffic capacity is arranged by means of small micro cells. As a mobile station MS moves from one cell to another, handover to a new radio cell is performed on the basis of specific handover criteria. The aim is to perform handover with minimum interference to the call in progress. However, problems arise with micro cells with small cell coverage; in them, handover must be performed fairly frequently, which increases the risk of failure in handover.
The mobile station MS and the base transceiver station BTS serving it continuously measure the level and the quality of the signal on a radio connection, for instance in case of any handover or power adjustment. The mobile station MS measures the signals from the base station BTS serving it and from the base stations BTS closest to its area of location, regularly transmitting the results of the measurement as a report message through the base transceiver station BTS serving it to the base station controller BSC. A base station BTS performs measurements of the level and quality of the signal on ongoing radio connections and possibly also other measurements indicating the general level of interference in the network.
When the level and/or quality of the signal on an individual radio connection drops below the desired level, adjustment of the transmission power can be performed at a base station BTS or/and in a mobile station MS for improving the radio connection. The transmission power of the mobile station MS is usually adjusted from a fixed network by means of a specific power adjustment algorithm. The mobile station MS measures the reception level (field strength) and the quality of a downlink signal received from the base station BTS1 of the cell serving it, and the base station BTS1 of the cell serving the MS in turn measures the reception level (field strength) and the quality of an uplink signal received from the mobile station MS. On the basis of these measurement results and the power adjustment parameters set, a power adjustment algorithm defines a suitable level of transmission power which is then announced to the mobile station MS in a power adjustment command. Power is adjusted continuously during the call. The problem with increasing transmission power is that it increases the level of interference in the network, for which reason the transmission power is to be kept as low as possible.
In digital Time Division Multiple Access TDMA radio systems, a group of mobile stations MS can use on a time-division basis the same carrier frequency, or radio channel, for communicating with a base station BTS. The carrier wave has been divided into successively recurring frames which are further divided into time slots, e.g. into eight time slots which are allocated to users according to need. Short data bursts are transmitted in the time slots. From the point of view of the network, eight traffic channels, for instance, can thus be created on a single carrier wave.
In a conventional TDMA system, each mobile station is allocated one traffic channel time slot for data transmission. According to the available bandwidth and the channel coding used in transmission, the maximum transmission rate on a single traffic channel is restricted at a fairly low level, being 9.6 kbit/s or 12 kbit/s in a GSM system. During the past few years, need for high-speed data services in mobile communication networks has increased significantly. One way of increasing transmission rate is to use more than one channel pair, which means that several TDMA time slots in a frame are allocated for a single mobile station. At the transmission end, a high-speed data signal is divided between such parallel channels for the duration of the transfer over the radio path, and then coded again at the receiving end. This enables offering data transmission services with a multiple transmission rate, depending on the number of channels used, as compared with the conventional transmission rate.
The operation of a Code Division Multiple Access CDMA in radio systems is based on spread-spectrum transmission. The data signal to be transmitted is multiplied by a subscriber specific spreading code, which spreads the transmission on a broadband radio channel, which is typically 1.25 MHz. This allows several users to send simultaneously on the same broadband radio channel CDMA signals processed with different spreading codes. At the receiving end the CDMA signal is decoded with a subscriber spreading code, which produces a narrowband data signal. The broadband signals of the other subscribers appear as noise in the receiver as compared with the signal desired. In a CDMA system a subscriber specific spreading code thus produces a system traffic channel in the same sense as a time slot in TDMA systems. In CDMA systems data transmission rate can also be increased by allocating several traffic channels, or spreading codes, to the sub-scriber.
The problem with rapid transmission using several traffic channels is that it reserves an extensive amount of transmission capacity in a cell. If several subscribers using rapid data transmission are simultaneously in a network cell, access to the network by ordinary users may be blocked due to cell overload.
The aim of the present invention is to provide a mobile station in a digital data communication system with a service level needed, without interfering with the normal operation of the network.
Setting of this new type of service level is achieved by a method of the invention which is characterized in that a network interference load is defined and that, in connection with the establishment of a connection, transmission parameters are set at a base station and in a mobile station according to the network interference load.
The invention further relates to a method for setting a service level in a digital mobile communication system, the method being characterized in that a network interference load is defined and network parameters are set according to the network interference load.
An object of the invention is also to provide a digital mobile communication system which is characterized in that it comprises means for defining network interference load; means for defining transmission parameters for a connection in response to the network interference load; means for setting transmission parameters at a base station in response to the output of said means defining the transmission parameters; and means for setting transmission parameters in a mobile station in response to the output of said means defining the transmission parameters.
The invention further relates to a digital mobile communication system which is characterized in that it comprises means for defining network interference load and means for setting network parameters in response to the network interference load.
The invention is based on the idea that, with the general load and/or level of interference in a network permitting, individual mobile subscribers can be offered, when necessary, better than standard service.
The method of the invention defines network interference load e.g. on the basis of the load of the network cells and/or the level of interference measured at base stations and/or the signal quality. Transmission parameters for a connection are set on the basis of the interference load defined. For instance, low interference load enables reducing channel coding of the data transmitted on the radio path, increasing transmission power at a base station and/or in a mobile station or allocating several traffic channels to a mobile station according to the specific needs of each mobile subscriber, without impairing the general service level of the network. Network parameters can also be set during low interference load. For instance, network handover criteria can be set in such a way that the cell coverage is extended.
One advantage in setting service level in this way is that it enables increasing the number of subscribers served when the general level of interference in the network is low.
Another advantage of the method is that transmission errors in high-speed data transmission are avoided because channel coding is not reduced in an environment subject to interference.
A further advantage of the invention is that it allows increasing the signal transmission rate of a mobile station when there are only a few subscribers and the level of interference is low.
In addition, the method of the invention also has the advantage that, with the interference load of a network permitting, a stronger signal and thus a better than standard signal/interference ratio can be offered to individual mobile subscribers.
A further advantage the method of the invention has is that in an environment of low interference level, the number of handovers can be reduced by increasing the size of a cell or cells.